Presently, various resins are used to produce numerous types and styles of pipe. For example, polyethylene resins have long been used to produce high stiffness pipe used in water, gas, and other fluid transport applications. Polyethylene pipe classified as PE-100, MRS 10, or ASTM D3350 typical cell classification 345566C is especially desirable for use under conditions requiring higher pressure ratings. The higher pressure ratings are due in part to the pipe's higher stiffness. To obtain a PE-100 classification, PE-100 pipe is required to meet certain standards specifying stiffness, resistance to chemical attack, and brittleness, as expressed as rapid crack propagation under cold temperature environments or applications. Further, such pipe must meet a deformation standard which is determined under pressure at elevated temperatures.
Although certain PE-100 classified polyethylene resins are commercially available, these polyethylene resins are the “bimodal” type. Bimodal polyethylene resins are made in reactor systems involving two or more reaction zones or by blending two or more distinct polymers. Unfortunately, despite the excellent toughness of these resins, bimodal resins are found to be deficient when pipe is formed by extrusion under certain conditions. Typically, bimodal resins perform adequately in the production of standard, small diameter pipe, that is, pipe having diameters between 1-12 inches. However, when forming large diameter and/or thick-walled pipe, the bimodal resins begin to “sag” or “slump” as the newly formed pipe emerges from the extrusion die. Apparently, the still semi-molten polymer does not have sufficient melt strength during the short time before the pipe cools and hardens to resist deformation due to gravitational force. This phenomenon causes an inconsistency in the pipe which results in the lower portion of the pipe to be thicker and heavier than the upper portion. A pipe manufacturer can compensate to some degree by adjusting the die to form pipe that is “out of round” as it exits the die, so that it subsequently sags back into a better balance. However, such adjustments are complicated, and the degree to which the pipe manufacturer can compensate for “sag” is limited. As a result, currently available PE-100 classified resins are typically used only to produce small diameter pipe. Accordingly, there is a need for a PE-100 classified resin which can be employed to produce both small and large diameter PE-100 pipe.
During the production of high density olefin polymers or resins, such as high density polyethylene, conventional supported chromium catalyst systems may be employed. However, as originally commercialized, use of these conventional supported chromium catalyst systems are limited. Typically, the conventional supported chromium catalyst systems are used in solution polymerization processes. A more economical route to produce many grades of commercial olefin polymers employs a slurry process. In the slurry process, the polymerization reaction employs one or more monomers in the presence of a diluent and the catalyst system. Beneficially, the polymerization reaction occurs in the slurry process at a temperature which is sufficiently low to permit the resulting polymer to be largely insoluble in the diluent. However, as explained below, conventional supported chromium catalyst systems are undesirable for use in a slurry process to produce a PE-100 classified resin. Accordingly, there is a need for a supported chromium catalyst system which may be employed in a slurry process to produce a PE-100 classified resin.
Currently, mono-1-olefins, such as ethylene, are polymerized with catalyst systems employing transition metals such as titanium, vanadium, chromium, or other metals, either unsupported or on a support, such as alumina, silica, aluminophosphate, titania, zirconia, magnesia, and other refractory metals. Such catalyst systems are used to form homopolymers of ethylene. Additionally, comonomers such as propylene, 1-butene, 1-hexene, or other higher mono-1-olefins may be copolymerized with ethylene to provide resins tailored to specific end uses.
Polymers having broad molecular weight distributions and improved physical properties, such as environmental stress crack resistance (ESCR), slow crack growth resistance (PENT), and impact resistance, can be formed by employing chromium catalyst systems containing aluminophosphate supports, to include alumina (Al2O3). For example, polymers having a molecular weight distribution (Mw/Mn) of up to 30 can be obtained with aluminophosphate supported catalyst systems. Aluminophosphate supports are characterized by the amount of phosphate in the support, or more precisely, by the phosphorous to aluminum molar ratio (P/Al) of the composition. The P/Al molar ratio can vary from 0, e.g. alumina, to 1 for stoichiometric aluminum phosphate (AlPO4). At a P/Al molar ratio of 1, a crystalline solid of very little surface area and minimal pore volume is obtained. As a result, the activity of catalyst systems employing an aluminophosphate support having a P/Al ratio of 1 is minimal. Additionally, catalyst systems employing chromium supported on alumina also have a very low activity. Therefore, in practice, the commercially preferred P/Al molar ratio of phosphorus to aluminum in chromium/aluminophosphate catalyst systems is 0.7 to 0.9. See The Structure of Coprecipitated Aluminophosphate Catalyst Supports; T. T. P. Cheung, K. W. Willcox, M. P. McDaniel, and M. M. Johnson; Journal of Catalysis, Vol.102, p.10-20 (1986).
Below a P/Al molar ratio of 0.3, the activity is considered too low to be practical. Yet, catalyst systems employing aluminophosphate supports having such lower P/Al molar ratios produce polymers which desirably have the broadest molecular weight distributions, and thus, the highest ESCR and impact resistance values. Another disadvantage of conventional chromium/aluminophosphate catalyst systems is that they incorporate certain comonomers, such as 1-hexene, very poorly. In conventional processes, 1-hexene can kill the activity of chromium/aluminophosphate catalyst systems. Thus, while conventional chromium/aluminophosphate catalyst systems are excellent for producing very high density blow molding resins, such catalyst systems have not gained acceptance for the commercial production of lower density copolymers which are used to produce film and pipe.
Despite existing catalyst systems, methods, and resins, a need exists for improved catalyst systems, methods, and resins for producing PE-100 pipe. It is to the provision of catalyst systems, methods, and resins that meet these needs that the invention is primarily directed.
This invention also relates to polyolefin compositions, methods for producing polyolefin compositions, and to processes for using polyolefin compositions for producing pipe. More specifically this invention relates to the production of PE-100 classification polyethylene pipe and the resin used to make that pipe.
This invention relates to the polymerization and copolymerization of a mono-1-olefin, such as ethylene, with a higher alpha-olefin comonomer, such as 1-hexene.
This invention further relates to producing cost-effective ethylene polymers and copolymers of exceptional toughness, which can be processed into large diameter pipe without the distortion caused by the commonly observed “sag” or “slumping” phenomenon during extrusion.
The present invention describes a new type of PE-100 polyethylene resin which does not exhibit this slumping behavior, and which can, for the first time, be used to produce very large diameter pipe without problem. The present invention also describes the unique catalyst system that is used to produce this polymer in a single reaction zone, which means that the resin can be manufactured more efficiently that the common “bimodal” type resins.